Quoted from Anth:A pinball cab isn't air tight so putting in more holes wouldn't accomplish anything.
I know it isn't airtight. But it is, for the most part, still a closed box, equipped with a low power amp, a small woofer. You would hear some of the back waves coming out of a second hole in the box, no doubt. When I open the coin door on my games I hear the sound spilling out...dramatically louder. Usually I have to close the coin door, listen again, and readjust the volume.
Any other explanations for the hole? If I saw a picture, might be able to suggest from the dimensions if it was put there for a speaker, perhaps a mistake, a cab intended for another game or something along those lines.